Modifiability of antenna structure is a preferable characteristic in communications devices designed to be used in more than one radio system. Such systems include e.g. the AMPS (Advanced Mobile Phone System), GSM900 (Global System for Mobile Telecommunications), DCS (Digital Cellular System), GSM1800, GSM1900, WCDMA (Wideband Code Division Multiple Access) and UMTS (Universal Mobile Telecommunication System). An antenna may be construed so as to have two separate operating bands which cover the frequency ranges used by the different systems, or so as to have a single, relatively wide, operating band which covers the frequency ranges of at least two systems. In the latter case there is, however, the risk that the antenna characteristics are not satisfactory e.g. in part of the wide operating band. This drawback is avoided if the resonance frequency of the antenna can be electrically shifted so that the operating band falls into the frequency range of the currently used system.
From the prior art it is known an electrical adjustment method for an antenna, where the reactance generated by capacitors or coils connected to a monopole antenna, for example, can be changed by means of electronic switches. As the reactance changes, so do the electrical length and resonance frequency of the antenna. A drawback of this method is that the arrangement calls for extra components.
From the publication JP 8242118 it is known a solution according to FIG. 1. It comprises a planar radiating element 110 with two openings, such as openings 111 and 112, at each side of the element, extending from the edge of the element towards the center area thereof. To each opening an electronic switch is connected which, when conducting, shorts the opening in question at a certain point. For example, switch SW1 can be used to short-circuit opening 111 relatively near the mouth of the opening, and switch SW2 can be used to short-circuit opening 112 approximately at the middle of the opening. Changing the state of a switch changes the electrical dimensions of the radiating element and, thereby, its resonance frequency. Each switch is controlled with a control signal of its own, such as C1 for switch SW1, so the antenna can be adjusted at relatively small steps. The disadvantage of this solution is the extra cost caused by the quantity of switch components and their mounting.